Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, touch sensor panels, joysticks, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device that can be positioned behind the panel so that the touch-sensitive surface can substantially cover the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Touch sensor panels can be formed from a matrix of drive and sense lines, with sensors or pixels defined, in some embodiments, by where the drive and sense lines cross over or come close to each other while being separated by a dielectric material. Drive or transmitting logic can be coupled to the drive lines, and sense or receiving channels can be coupled to the sense lines. During a scanning process, the drive logic can drive each drive line with a stimulation signal, and the sense channels can generate sense data indicative of the amount of charge injected into the sense lines due to the stimulation signal. A panel processor can identify touch locations based on the sense data, because the amount of charge is related to the amount of touch.
However, the voltage required by the drive logic for providing the stimulation signal can be much higher than the voltage required by the sense channels for sensing the injected charge. This can force the drive logic and sense channels to be implemented in discrete chips, causing the sensor panel circuitry to be larger in size and more expensive.
Further, involvement by the panel processor in the scanning process can occupy a significant amount of time, increasing the processing burden of the panel processor beyond that which is necessary to identify an occurrence or absence of a touch event based on sense data generated from the scanning process. This significant amount of processing time can make a processor too busy to perform other functions and can slow down devices using a sensor panel. Additionally, processors typically consume a significant amount of power during operation, which can be particularly problematic when a sensor panel is used in conjunction with a hand held device, as many hand-held devices have a limited power supply.